When they were playing around with their SearchWiki system, there was an 'X' next to all results that you could click to make them go away, which I was pretty excited about, until I realized it didn't actually permanently banish that site; just removed it from the current results page.
There are a bunch of greasemonkey scripts to let you blacklist specific sites from Google results, but relatively frequent changes to Google's results page seem to make them always fall behind, so it's hard to find a consistently working one. This is the best-maintained one I can find, and it's actually not currently working: http://userscripts.org/scripts/show/44418
There are a bunch of greasemonkey scripts to let you blacklist specific sites from Google results, but relatively frequent changes to Google's results page seem to make them always fall behind, so it's hard to find a consistently working one. This is the best-maintained one I can find, and it's actually not currently working: http://userscripts.org/scripts/show/44418